1. Field of the Invention
The present invention relates to a polarization-independent demultiplexing device and an optical communication system, such as a wavelength or frequency division multiplexing communication system, using this demultiplexing device. A stable received output can be obtained, even if the polarization state of an input fluctuates, by an optical device for demultiplexing signals multiplexed in wavelength division multiplexing transmissions or the like.
2. Description of Related Background Art
Conventionally, several kinds of optical devices for demultiplexing wavelength-division multiplexed optical signals have been proposed. Those optical devices are roughly classified into a bulk-type device for demultiplexing optical signals using a device, such as a grating, and a waveguide-type device which includes a basic device, such as a directional coupler. In particular, the degree of freedom in design of the waveguide-type device is greater than that of the bulk-type device, and the wavelength selection function of the waveguide-type demultiplexing device can be controlled. Therefore, the wavelength selection width of the waveguide-type device is narrower than that of the bulk-type device, and hence the wavelength multiplicity of optical transmission can be increased in the waveguide-type device. Further, the center selection wavelength of the waveguide-type device, such as a distributed feedback (DFB)-laser diode (LD) filter, can be readily controlled, and thus the tunability characteristic of the waveguide-type device can be effectively utilized.
However, in the waveguide-type device, the polarization dependency of device performances (i.e., a difference between performances for TE (transverse electric) and TM (transverse magnetic) modes) is strong. Therefore, in optical fiber transmissions, the output of demultiplexed light from the waveguide-type demultiplexing device, which is obtained after light transmission through an optical fiber, fluctuates largely due to fluctuation in the polarization state of the light input into a receiver device. Thus, the light transmission characteristic is unstable. As shown FIG. 1, when a light wave 102 at wavelength .lambda..sub.1 is demultiplexed from a light input 101, which contains a component of wavelength .lambda..sub.1 and a component of wavelength .lambda..sub.2, by a waveguide-type demultiplexing device 100, that is a device having characteristics for demultiplexing a light wave of wavelength .lambda..sub.1, in a TE state, a component of wavelength .lambda..sub.1 is also mixed into an output 103 of the waveguide-type demultiplexing device 100 and the demultiplexed output 102 fluctuates if the polarization state of the component at wavelength .lambda..sub.1 in the light input 101 varies.
To solve that problem, there is a method in which the light wave transmitted through the optical fiber is caused to be transmitted through a polarization control device 110 before being input into the waveguide-type demultiplexing device 100, as shown in FIG. 2, to control its polarization state (TE or TM) as desired. In this connection, reference should be made to a component of wave length .lambda..sub.1 in an output 111, which has been converted to the TE state. Thus, the light wave can be stably demultiplexed into two wavelength-component outputs 112 and 113 by the waveguide-type demultiplexing device 100. In the case where the polarization control device 110 is used, however, means for adjusting the light wave to an optimum polarization state in accordance with fluctuation in the light transmission line, such as an optical fiber, is needed (i.e., a polarization diversity function or the like is required), and the resulting structure becomes quite complicated. Furthermore, in the case of wavelength division multiplexed signals, input polarization states of respective wavelength components vary when the multiplexed signals are transmitted through the optical fiber and input into a receiver unit, so the polarization control device 110 should be separately prepared for the respective multiplexed wavelengths. Thus, the number of components and insertion loss increase.